Focused ion beam apparatuses have been known as apparatuses for observation, various evaluations, analyses, and the like on samples of semiconductor devices or the like and for fabrication of transmission electron microscope (TEM) samples made from fine thin samples extracted from the samples and fixed on sample holders.
This type of focused ion beam apparatus includes an ion source emitting ions. The emitted ions are formed into a focused ion beam (FIB) for irradiation.
Some types of ion sources are known, including plasma ion sources and liquid-metal ion sources. As an alternative ion source that can produce a focused ion beam with a higher luminance and a smaller beam diameter than the above-mentioned counterparts, a gas field ion source (GFIS) has been known (see JP-A-H07-240165, for example).
A gas field ion source mainly includes an emitter with a tip portion sharpened into atomic level, a gas source that supplies helium (He) or other gas to the periphery of the emitter, a cooling unit that cools the emitter, and an extracting electrode spaced apart from the tip portion of the emitter.
This configuration, after supply of gas, applies an extracting voltage between the emitter and the extracting electrode and cools the emitter. As a result, the gas is ionized into gas ions due to a high electric field at the tip portion of the emitter. The resulting gas ions repel at the emitter maintained at a positive electric potential and are extracted toward the extracting electrode. The gas ions then are then converged into a focused ion beam, while being accelerated moderately.
In particular, ions emitted from a gas field ion source have a higher luminance and a smaller light-source diameter as described above and also have a smaller energy spread. Therefore, the ions emitted from the gas field ion source can be applied on samples with their beam diameter highly focused. This enables higher-resolution observation and finer etching, for example.
To produce a focused ion beam with a small beam diameter, the emitter preferably has a pyramid crystal structure at its tip portion whose leading edge is arranged with the smallest possible number of atoms. This structure enables gas to be ionized locally to produce gas ions, thereby producing a focused ion beam with a small beam diameter. For this reason, the tip portion of the emitter needs to have this crystal structure maintained in a constantly stable manner.
However, the crystal structure at the tip portion of the emitter can be easily broken and deformed from the original state. To address this, a method (treatment) is known for restoring the crystal structure at the tip portion of the emitter to the original state.
Specifically, the tip portion of the emitter is heated up to about 700° C. to 900° C., for example, whereby the atoms are rearranged and the crystal structure is restored to the original state. This treatment is performed periodically or as required for rearrangement of the atoms, so that the crystal structure at the tip portion of the emitter can be restored to the original state.
As described above, the crystal structure at the tip portion of the emitter ideally has atoms in a regular pyramid arrangement. However, a practical method for arranging atoms in an ideal pyramid without fail has not been established yet. Related-art emitters thus have a structure with a large number of atoms arranged at their leading edges.
Reliable reproducibility may not be provided by the above-described treatment performed for the rearrangement of atoms because of the large number of atoms arranged. The crystal structure remains incomplete in many cases, resulting in low yield of thermal restoration.
Because of the large number of atoms, a long heating time is typically required for their rearrangement. This means, even if the atoms are successfully rearranged, the tip shape of the emitter is likely to grow large because of thermal effects. The optimum value of the extracting voltage to be applied to the emitter, as a result, inevitably increases every time the treatment is performed as shown in FIG. 19.
In particular, a rise in the optimum value of the extracting voltage can increase load on the emitter and render the emitter easily damaged through electric discharge or other factors. As the voltage approaches the output limit of the extracting voltage of the apparatus, the emitter can no longer be used, which results in a shorter life of the emitter.